Stabilization of alkali metal permanganate in alkaline solution

ABSTRACT

Stabilization of alkali metal permanganate against deoxidative, self-decomposition in aqueous, alkaline solution by preventing ultrafine (colloidal) MnO2 dispersed in said solution from agglomerating by providing therein small amounts of anionic anticoagulating agent for dispersed MnO2, preferably perfluorocarbon surfactant.

nie :11 States Patent Posselt 1 Mar. 28, 1972 STABILIZATION OF ALKALI METAL IPERMANGANATE IN ALKALINE SOLUTION Inventor: lllans S. Posselt, La Salle, Ill.

Assignee: Carus Corporation Filed: Mar. 1, 1968 Appl. No.: 709,816

252/186, 252/313 Int. Cl. ..C02b 5/02, Cl 1d 1/28 Field 01' Search ..252/156, 80, 103, 186; 23/58 References Cited UNITED STATES PATENTS l/l956 Brice et a1 ..252/71 X 2,750,334 6/1956 Brown ..204/32 X 3,000,829 9/1961 3,210,284 10/1965 Duvall ..252/80 Primary Examiner-Herbert B. Guynn Assistant Examiner-Arnold I. Rady Attorney-Marzall, Johnston, Cook & Root [5 7] ABSTRACT 7 Claims, No Drawings STABILIZATION OF ALKALI METAL PERMANGANATE IN ALKALINE SOLUTION BACKGROUND OF THE INVENTION AND PERMANGANATE CHEMISTRY Alkaline solutions of potassium permanganate are used in a variety of industrial applications such as metal cleaning, radioactive decontamination, metal descaling, gas scrubbing, etc. In all these applications the longevity of the KMnO, is of high economic importance.

The useful life of a permanganate bath is not only determined by the rate of consumption, e.g., due to oxidation of lower oxides (as applies to metal descaling), but also by parasitic side reactions which result in the useless evolution of molecular oxygen from the bath. It is the principal subject of this invention to have devised measures to suppress said side reactions and thus to substantially lengthen the useful life of an alkaline permanganate bath. In view of the relatively high cost of replenishing KMnO the invention represents a significant economic improvement in this area of technology.

To understand the principles of this invention, it is necessary to appreciate the pertinent chemistry:

To illustrate the useful chemical action of KMnO. let us look at how it typically works in scale removal from a metal surface. A typical oxide scale will consist of higher oxides in its outer layers and of lower oxides in its inner layers (i.e., at and close to the interface metal-metal oxide). The action of MnO in scale removal has been shown to consist of an oxidative attack upon the lower oxide layers of the scale, causing a change in its physical and chemical properties which will lead to either partial dissolution of the scale or more susceptibility to pickling in a subsequent acid treatment step.

Thus, the useful chemical action exerted by KMnO, in this alkaline system can be expressed in a simplified manner in the following equation (using steel as the metal under treatment):

2Fe0 ZKMnO, ZKOH Fe 2K MnO H Ou Under the conditions of this type of a bath (alkali concentration in the range from g./l. to 150 g./l. NaOH and temperature up to 220 F.) permanganate ion (MnOf) is the only effective species in metal descaling, while manganate ion (M- nOf') is without effect.

Aside from the useful chemical reduction of KMnO, as exemplified by equation (I), permanganate in alkaline solution will also undergo self-decomposition to manganate, in a reaction generally referred to as deoxidation.

4KMnO 4KOH I. 4K MnO, 0 2I-I,O (II) The significance of this reaction is that molecular oxygen is generated from permanganate without any economic benefit. To prevent or curb this self-decomposition is of high interest to the economics of the process.

The reaction mechanisms proposed for the above mentioned deoxidation reaction are quite complex and up to this date none of them is fully satisfactory. Nevertheless, it has been established that reaction (II) represents a quasi-equilibrium (not a true equilibrium because the O continuously escapes from the system). The quasi-equilibrium state is characterized by the fact that reaction (II) will come to a virtual standstill when a certain concentration of manganate is present in the system. Accounts of results of investigations into this reaction have been published by Landsberg et al.

In addition to reaction (II) there are still other reactions occurring in the system. A very significant one in this context is the hydrolysis (or disproportionation) of K MnO This reaction is important because it can disturb the (quasi) equilibrium in reaction (II), causing more KMnO to be decomposed into K MnO and 0 Reaction (III) has been said to occur in both directions, but many investigators were unable to confirm the hypothesis that KMnO will consistently react with MnO to any significant extent to give K MnO Thus to the present date it remained a question whether and under what conditions reaction (III) can attain the state of equilibrium.

Tomicek et al. (Collection Czech. Chem. Commun. 11, 44958(l939) have described the usefulness of telluric acid for stabilizing quadrivalent manganese in analytical applications of permanganate. The stabilizing action of telluric acid is explained in terms of a complexing power of tellurate ion on quadrivalent manganese. This approach, however, does not appear very feasible for industrial purposes since tellurium compounds are rather expensive and furthermore toxic.

In applications where KMnO is the only active species, e.g., metal descaling, the removal (and deposition) of MnO is highly undesirable since it lowers the concentration of K M- n0. which in turn induces further deoxidation (loss of active oxygen). In the light of this theory, K MnO can be regarded as a deoxidation inhibitor.

DESCRIPTION OF INVENTION The overall process of such caustic permanganate solutions can be visualized by the following reaction scheme:

(Ill) loss of O2; deoxidation KMn0-----+ repeat itself.

From the net reaction of such systems:

4KMnO, 2I-I O 30 4Mn0 4KOI-I (IV) and the fact that molecular oxygen is produced by deoxidation, one can easily deduce that deoxidation is a useless and wasteful process. Further, it is very desirable to prevent or suppress the deposition of MnO, on solid reactant surfaces since rates of oxidation slow down by physically masking attack on the reactant.

The two reversible reactions of Equation III, wherein alkali metal manganate is a component, are expressed more fully in Equation V, below, wherein the horizontal equation represents the deoxidation reaction and the vertical equation represents the disproportionation (hydrolysis) reaction. Hi0 KMI'IO KOH If: M1104 E20 OzT KOH KMnOc MnOz M) (Colloidal) (Coagulated) Having developed the above overall reaction scheme of the interrelated deoxydation reaction and disproportionation reaction and having deduced therefrom that coagulation of MnO consumes alkali metal manganate in the disproportionation reaction, which in turn results in wasted alkali metal permanganate consumption in the deoxidation reaction, my discoveries are put into practical application by providing in alkaline, alkali metal permanganate solutions chemical means for inhibiting the disadvantageous, irreversible coagulation of colloidal MnO into insoluble MnO macrosize aggregates.

The latter is obtained by addition of compounds acting as anti-coagulating agents or protective colloids for ultrafine or colloidal MnO in the solution. These compounds are anionic and comprise, as a general class, surfactants, polyelectrolytes or protective colloids. They must be stable toward both alkali and the powerful oxidative properties of the alkali metal per manganate in aqueous solution, especially at elevated temperatures.

The group of compounds best suited for the above purposes are anionic, saturated fluorocarbon surfactants, particularly saturated fluor ocarbon sulfonic acids ant l 55k? thereof, par ticularly alkali metal salts, and saturated fluorocarbon carboxylic acids and salts thereof, particularly alkali metal salts. These sulfonic or carboxylic acids have the general formula RSO H or RCOOl-l wherein R is a saturated fluorocarbon group having 1-18 carbons and particularly as described in- U.S. Pat. No. 2,732,398. As therein described, the group R is a saturated fluorocarbon structure containing l-l8 carbon atoms, each of which is perfluoroalkyl and perfluorocycloalkyl (perfluorocyclohexyl). For the purposes herein, the group R preferably has at least three carbons.

The invention embraces a dry mixture of said alkali metal (i.e., sodium and/or potassium) permanganate and 0.01 to 1 percent by weight, based on said permanganate, of the aforedescribed anionic surfactants or MnO anti-coagulating agents. Other compounds may be mixed therewith if desired. For example, the dry mixture may contain alkali metal hydroxide or alkali metal hydroxide and alkali metal carbonate to provide the desired alkalinity of a metal descaling bath.

These dry mixtures may thus comprise:

a. KMnO, or NaMnO, and 0.01 to 1 percent of the anionic surfactant.

b. KMnO, or NaMnO.,, and in percentages based on the permanganate, NaOH or KOH- 50 250%; Na CO -O-250%; anionic surfactant 0.01 to 1 percent.

EXAMPLE 1 An example of such dry mixture is:

Present by weight KMnO. 30 1 NaOl-l 60 Na CO, (optional) Perfluoroalkyl sulfonic acid or its alkali metal salt 0.03%

The preferred perfluoroalkyl sulfonic acid has at least four carbons and may be, for example, perfluoroheptane sulfonic acid or perfluorooctane sulfonic acid. The alkali metal salts thereof may be used, e.g., the sodium or potassium salts. The acid will be neutralized to the alkali metal salt, in any case, in the ultimate alkaline permanganate solution. Instead of the ;perfluorosulfonic acid in the above example, the anionic sur- 1 factant may be a perfluoroalkyl carboxylic acid with at least four carbons or the alkali metal salt thereof, e.g., peri fluoroheptanoic acid, perfluorooctanoic acid or their sodium 1 or potassium salts. The anhydrides of the aforesaid perfiuoroalkyl carboxylic acids are other alternatives for the surfactant of the dry mixtures recognizing, .of course, that such anhydrides will be hydrolyzed into the corresponding acids in the form of their alkali metal salts in the aqueous, alkaline permanganate solutions.

The above described dry mixtures may be dissolved in water in the proportions of about -250 grams dry mixture per liter of water. An optimum balance between useful bath life and oxidative efficiency, particularly for metal descaling, for the mixtures substantially corresponding to the proportions of Example 1 is achieved at a solution strength of 160-180 grams dry mixture NaOl-l liter of water, corresponding to an alkalinity in the order of about 93 to 107 grams NaOl-l.

In terms of ranges of proportions of these dry mixtures, particularly those ultimately to be used inaqueous metal descaling baths, the alkali metal permanganate (preferably KMnO may constitute 5-95 percent by weight of the mixture; the alkali hydroxide (preferably NaOl-l), 5-95 percent by weight; ,the optional alkali metal carbonate (preferably Na CO 0-60 percent by weight; and the anionic surfactant ,(preferably the aforedescribed perfiuorocarbon compounds), 0.01-1 percent by weight.

The aqueous solutions, particularly those for metal descaling, may comprise in the preferred concentration ranges: 20-150 grams per liter of alkali metal hydroxide, preferably NaOl-l; 5-100 grams per liter of alkali metal permanganate,

preferably KMnO 0-100 grams per liter of alkali metal carbonate, preferably Na CO and 25-100 milligrams per liter of the above described anionic surfactant. in the initial makeup of such solutions or the replenishment thereof in use such as metal descaling, the solute ingredients may be added together in the form of the above described dry mixtures or separately as individual components thereof.

In metal descaling of ferrous metals, e.g., steel plates, wires or the like, made from mild steel, tool steel, carbon steel, stainless steel or other ferrous alloys, the baths preferably are arranged in the sequence of a permanganate, alkaline, scaleconditioning bath, a water rinse, an acid pickling bath, and a water rinse. Exemplary scale conditioning baths have a KMnQ, concentration of 2-10 percent, an NaOH concentration of 5-10 percent; an anionic surfactant concentration of 25-100 mg. per liter; and an elevated bath temperature of 190-230 F. During use, the bath is replenished as required to maintain the above optimum concentrations. In addition to stabilizing the bath against deoxidation loss of KMnO via maintaining the colloidal state of MnO in the bath, the above described perfluorocarbon anionic surfactants also serve in suppressing evolution of caustic mist from the bath and in reducing drag out loss of the bath on treated metal parts removed therefrom.

Duvall US. Pat. No. 3,210,284 states that KMnO can be stabilized in caustic solutions by addition of Ba ion which precipitates all manganate (VI) as the insoluble barium salt. Since manganate (V1) is inevitably the first state of KMnO reduction in caustic solution (p1-1 11) one can-if Ba is present-utilize the exchange of one electron only IMHOF 8 v M:

Mn04= Ba BaMnO, (insoluble) as opposed to the usual 3-electron exchange in the absence of barium ion:

MnO,,-+ 3e 411* MnO 2H O Thus, KMnO, oxidations carried out under caustic conditions and in the presence of barium ion have a profound economic disadvantage which entirely overshadows other possible benefits gained from barium ion addition.

Accordingly, in the practice of the invention herein, a substantial amount of barium ion in the permanganate solutions is not considered to be desirable. Also, the presence in said solutions of substantial amounts of other alkaline earth metal ions :such as Ca and Mg ions (and also heavy metal ions) has an overall detrimental effect by promotion of MnO precipitation, thus increasing permanganate loss through the above described permanganate self-decomposition.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the forms herein disclosed being preferred embodiments for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

1. A composition consisting essentially of alkali metal permanganate, and, as a stabilizer against self-decomposition of said permanganate in aqueous, alkaline solution, about 0.01 to 1 percent by weight, based on said permanganate, of a perfluoro alkyl compound selected from the group consisting of perfiuoro alkylsulfonic and carboxylic acids having 1 -l8 carbons and perfluorocyclohexylsulfonic and carboxylic acids which is stable in said alkaline alkali metal permanganate solu tion and acts as an anticoagulant for colloidal MnO in said solution.

2. A composition as claimed in claim 1 wherein said perfluoroalkyl compound is a perfluoroalkyl sulfonic or carboxylic acid having 4-18 carbon atoms or alkali metal salt thereof and said permanganate is potassium permanganate or sodium permanganate.

3. An aqueous, alkaline, alkali metal permanganate solution 7 having a permanganate concentration of about 5-100 grains per liter and an alkalinity equivalent to about 20-150 grams per liter of NaOH, said permanganate being stabilized against self-decomposition in said solution by about 0.01 to 1 percent by weight based on said permanganate, of a perfluoro alkyl compound selected from the group consisting of perfluoro alkylsulfonic and carboxylic acids having 1-18 carbons and perfluorocyclohexylsulfonic and carboxylic acids which is stable in said alkaline, alkali metal permanganate solution and acts as an anticoagulant for colloidal MnO in said solution.

4. A composition as claimed in claim 3 wherein said perfluoroalkyl compound is a perfluoroalkyl sulfonic or carboxylic acid having 4-18 carbon atoms alkali metal salt and said permanganate is potassium permanganate or sodium permanganate.

5. A method for improving stability of alkali metal permanganate against self-decomposition in aqueous, alkaline soluanti-agglomerating agent on said ultrafine manganese dioxide. 6. A method as claimed in claim -5 wherein said perfluoroalkyl compound is a perfluoroalkyl sulfonic or carboxylic acid having 4-18 carbon atoms alkali metal salt and said permanganate is potassium permanganate or sodium permanganate.

7. A process as claimed in claim 5 wherein said aqueous, alkaline, alkali metal permanganate solution has a permanganate concentration of about 5-100 grams per liter and'an alkalinity equivalent to about 20-150 grams per liter of NaOl-l. 

2. A composition as claimed in claim 1 wherein said perfluoroalkyl compound is a perfluoroalkyl sulfonic or carboxylic acid having 4-18 carbon atoms or alkali metal salt thereof and said permanganate is potassium permanganate or sodium permanganate.
 3. An aqueous, alkaline, alkali metal permanganate solution having a permanganate concentration of about 5-100 grams per liter and an alkalinity equivalent to about 20-150 grams per liter of NaOH, said permanganate being stabilized against self-decomposition in said solution by about 0.01 to 1 percent by weight based on said permanganate, of a perfluoro alkyl compound selected from the group consisting of perfluoro alkylsulfonic and carboxylic acids having 1-18 carbons and perfluorocyclohexylsulfonic and carboxylic acids which is stable in said alkaline, alkali metal permanganate solution and acts as an anticoagulant for colloidal MnO2 in said solution.
 4. A composition as claimed in claim 3 wherein said perfluoroalkyl compound is a perfluoroalkyl sulfonic or carboxylic acid having 4-18 carbon atoms alkali metal salt and said permanganate is potassium permanganate or sodium permanganate.
 5. A method for improving stability of alkali metal permanganate against self-decomposition in aqueous, alkaline solution which comprises preventing ultrafine manganese dioxide dispersed in said solution from aggregating by providing in said solution a small but sufficient amount of a perfluoro alkyl compound selected from the group consisting of perfluoro alkylsulfonic and carboxylic acids having 1-18 carbons and perfluorocyclohexylsulfonic and carboxylic acids which is stable in the alkaline, alkali metal permanganate solution and acts as anti-agglomerating agent on said ultrafine manganese dioxide.
 6. A method as claimed in claim 5 wherein said perfluoroalkyl compound is a perfluoroalkyl sulfonic or carboxylic acid having 4-18 carbon atoms alkali metal salt and said permanganate is potassium permanganate or sodium permanganate.
 7. A process as claimed in claim 5 wherein said aqueous, alkaline, alkali metal permanganate solution has a permanganate concentration of about 5-100 grams per liter and an alkalinity equivalent to about 20-150 grams per liter of NaOH. 